Currently, the mainstream type of transistor that is formed on a silicon substrate is a metal/oxide film insulator/semiconductor (MOS) type field effect transistor (MOSFET). As a method for improving the characteristics of such a MOSFET, a method of applying a tensile strain to an Si channel layer is reported in articles (J. Welser et al., “Strain dependence of the performance enhancement in strained-Si n-MOSFETs,” IEDM Tech. Dig. 1994, p.373. and K. Rim et al., “Enhanced hole mobilities in surface-channel strained-Si p-MOSFETs,” IEDM Tech. Dig. 1995, p.517). This is a method in which a thick SiGe buffer layer is grown on a silicon substrate, and the lattice of the SiGe buffer layer is relaxed, after which an Si channel layer is formed thereon. This is to increase the lattice constant of the SiGe buffer layer to be equal to that of unstrained SiGe so as to apply a tensile strain to the Si channel layer grown thereon.
In the Si channel layer in the hetero junction structure, which is subject to a tensile strain, the 6-fold degeneracy is lifted in the conduction band, and the band is split into 2-fold and 4-fold degenerated bands (Δ(2) and Δ(4)). At this time, the conduction band edge of the Si channel layer is formed by the 2-fold degenerated band Δ(2), and the effective mass of electrons when moving in the channel direction in the band Δ(2) is reduced. By forming an n-channel field effect transistor by using such a hetero junction, electrons can be confined in a hetero barrier of Si channel layer/SiGe layer.
Moreover, due to the reduction in the effective mass of electrons in the Si channel layer, the electron mobility is improved, and the operating speed of a transistor is improved. On the other hand, degeneracy is similarly lifted also in the valence band, and the valence band is split into a light hole (LH) band and a heavy hole (HH) band. At this time, the valence band edge in the Si channel layer is formed by a band of light holes, which are positive holes having a small effective mass, and the effective mass of light holes is smaller than the effective mass of holes in the SiGe layer. By forming a p-channel field effect transistor by using such a hetero junction, the hole mobility is improved due to a decrease in the effective mass of holes, thereby improving the operating speed of a transistor. Note however that since a hetero barrier is formed on the SiGe layer side, confinement of holes cannot be expected.
As described above, there have been reported that transistor characteristics are improved by applying a tensile strain to an Si channel layer both for n-channel and p-channel.
Problems to be Solved
However, in order to apply a tensile strain to an Si channel layer with the conventional method, it is necessary to grow the SiGe buffer layer to be sufficiently thick on the silicon substrate until the lattice is relaxed. When the lattice is relaxed, a large number of dislocations occur in the SiGe buffer layer. Moreover, a large number of dislocations inherently exist also in the Si channel layer formed thereon. Such dislocations not only deteriorate the characteristics of the transistor, but also present a problem in terms of long-term reliability. In view of this, it has been reported that the dislocations can be reduced by improving the structure of the SiGe buffer layer. However, a dislocation density of about 105 cm−2 is the limit at present. Thus, the device is very defective.
Moreover, such an SiGe buffer layer for lattice relaxation is required to have a considerable thickness (1 μm or more), whereby the crystal growth takes a long time, thus presenting a problem also in terms of throughput.